It has been established that some zirconia (ZrO.sub.2) sensors, made for the measurement of oxygen concentration in gases, can also be used to remove oxygen from these gases if suitably operated. The advantages of such materials have been recognized by Fouletier et al. in an article entitled "Electrically Renewable and Controllable Oxygen Getter," Vacuum, vol. 25, 1975, pp. 307-314 and an article entitled "Measurement and Regulation of Oxygen Content in Gases Using Solid Electrolyte Cells III Oxygen Pump-Gauge," J of Appl Electrochemistry, vol. 5, 1975, pp. 111-120. Although zirconia, usually doped with yttrium (Y.sub.2 O.sub.3) magnesium oxide (MgO) or lime (CaO), is the most extensively used material for these sensors, many other materials may be used, as cataloged by Subbarao in a paper entitled "Oxygen Sensors," Ferroelectrics, Vol. 102, 1990, pp. 267-280. Similarly doped hafnia (HfO.sub.2), thoria (ThO.sub.2), ceria (CeO.sub.2) and bismuth trioxide (Bi.sub.2 O.sub.3) are known oxygen ion conductors used for sensors. Semiconducting materials such as TiO.sub. 2 and Nb.sub.2 O.sub.3 can also be used for sensors.
In their normal operating condition, these sensors develop a voltage which is proportional to the logarithm of the ratio of the partial pressures of oxygen on both electrodes of the device. If one electrode is in contact with a low oxygen concentration, and the other electrode is in contact with a higher pressure of oxygen, then the latter electrode will develop a positive voltage with respect to the low oxygen concentration side. This is because negatively charged oxygen ions diffuse through the zirconia away from the high pressure side, leaving a positive space charge, which in turn sets up a field which balances the diffusion. In such a mode of operation of an oxygen sensor no current flows through the device, as the flow from diffusion and the internal electric field balance each other. The developed voltage (E) is described by the Nernst equation: E=RTln(p1/p2)/4F where R is the gas constant, T the absolute temperature, F Faraday's constant, and p1 and p2 the partial pressures of oxygen on both sides of the sensor.
If a voltage is applied of the same sign but larger than the Nernst voltage, then the device will start to draw current as oxygen ions are drawn from the low pressure electrode and flow towards the high pressure electrode. The electrodes are typically formed of platinum, which is gas permeable. Other gas permeable electrodes which may be used for sensors are known to include oxides such as SnO.sub.2, In.sub.2 O.sub.3 and oxides with a perovskite structure. The device works in essence as an oxygen pump, removing oxygen from the low pressure side and delivering it to the high pressure end. The oxygen which is removed may have been either free or bound to other atoms. This is termed the "extraction" mode. Oxygen atoms in the low pressure gas stream are converted at platinum molecules near the border of the low pressure electrode and the zirconia, called a "triple point" into oxygen ions, injected in the zirconia, transported through the wall, and delivered at another triple point between the high pressure platinum electrode and the zirconia where they are ejected as atoms through the electrode and into the high pressure gas stream.
Alternatively, if a voltage smaller than the Nernst voltage or of opposite sign is applied, then the current is reversed. Oxygen is now delivered from the high concentration side to the low concentration side. The pump is now working in what is termed the "injection" mode. In this case, the amount of oxygen injected can be very accurately derived from the electric current and Faraday's law. Each oxygen ion transported across the wall requires two electrons to be donated at the high pressure electrode. A gram-mole of oxygen requires 3.86.times.10.sup.5 Coulombs of current, since O.sub.2 gas requires four electron charges per molecule. To a very good approximation, the entire current is ionic in nature, and the electronic contribution to the current can be completely neglected.
In the case of oxygen extraction from a gas stream with an already low oxygen concentration, the current is not purely proportional to oxygen ions transported across the wall, especially for very small oxygen concentrations. In this case, especially at higher voltages, part of the ionic current may be due to zirconia decomposition, producing at first intermediate substances known as "black zirconia" and for high enough currents eventually leaving free zirconium (Zr).
Both the described pumping action due to the current and the black zirconia getter compound produced by material decomposition give rise to an extraordinary active and renewable getter material for oxygen. Somewhat less well known is the fact that these pumps can also decompose water. If the voltage is raised high enough (about 1.2 V for a typical cell at 650.degree. C.), then it is observed that water is decomposed. The oxygen is stripped from the water, leaving hydrogen as a residual in the original gas.
Combinations of oxygen sensors and oxygen pumps have been described, for example, in an article entitled "Electrolytic Removal of Oxygen from Gases by Means of a Solid Electrolyte," J. Appl Electrochemistry, 2, 1972, pp. 289-299. In this article, a gas which is to be cleaned or enriched with oxygen is treated in several steps with pumps for ejection or injection of oxygen.
The same system can also be used to supply a known partial pressure of oxygen to a stream of carrier gas having a known (possibly zero) oxygen concentration. For this purpose a controlled amount of oxygen is pumped into the gas with a known current, by decreasing or reversing the voltage as described above. Given a known flow rate of carrier gas in the inner tube, the concentration of oxygen can then be determined.
In a similar way, British Patent GB 2 256 045 A to Atkinson describes a system for injection of controlled amounts of moisture in a gas stream containing some amount of hydrogen. Oxygen is pumped into a gas containing a known amount of water and hydrogen, reacting with the hydrogen, and converting it into water at the platinum electrodes. At the typical temperatures and pressures used, the equilibrium for the hydrogen-oxygen reaction is entirely pushed towards the formation of water and, to a very good approximation, every oxygen ion transported through the wall gives rise to one water molecule. Thus, given the known initial water concentration, the resultant water concentration can be accurately determined.
For a similar reason, the presence of hydrogen in a gas stream complicates the detection of oxygen atoms. When an oxygen sensor is used to determine the concentration of oxygen in a gas stream, the reading which will be obtained is dramatically affected by the presence of hydrogen. All the hydrogen in the gas stream reacts at the sensor electrode with the available oxygen, and gives rise to an oxygen reading at the electrodes which is lower than that present in the original gas stream.
While many of these phenomena are known, they are not always easy to implement. For example, various gases may need to be sealed from each other aside from oxygen ion permeable zirconia walls and platinum electrodes. Yet it may be desirable to place a number of electrodes with different functions on a sealed inner surface without interference to the gas flow from electronic leads.
An object of the present invention is to provide a device that can conveniently perform a number of useful functions utilizing the above-described phenomena.
Another object of the present invention is to provide a device for which the initial concentration of oxygen gas or oxygen containing molecules within a gas need not be known in order to produce a known concentration of such molecules.
Another object of the present invention is to provide a system for measuring in a gas an unknown concentration of molecules containing oxygen atoms.
Yet another object of the present invention is to provide a system for removing oxygen from gases that contain molecules that react with typical oxygen removal systems and interfere with such removal.